Method of making a drawing steel sheet by continuous annealing process including shelf treatment therein

ABSTRACT

A soft steel sheet, in which C content is 0.02 to 0.10 percent, manufactured with a continuous annealing process comprising the following steps; heating the cold reduced steel strip up to the range of recrystallization temperature to 850*C, slow-cooling from said temperature to the range of A1 point to 600*C and then rapid-cooling from said temperature to the normal temperature at a cooling rate of 200*C/sec to 10,000*C/sec, and again heating up to the range of 300*C to 530*C and keeping at said temperature for at least 10 seconds, lastly cooling to the normal temperature. Such steel sheet is economical in cost because of high productivity.

United States Patent 11 1 Kubotera et al.

1 Oct. 1,1974

[73] Assignee: Nippon Kokan Kabushiki Kaisha,

Tokyo, Japan [22] Filed: Mar. 27, 1972 [21] Appl. No.1 238,459

[30] Foreign Application Priority Data 5/1970 McFarland et al. 148/12 4/1972 Kubotera et al. 148/2 OTHER PUBLICATIONS Richards, P. N.; Short-Time Annealing Characteristics of Capped, Rimmed, and Semi-Killed Wide Strip; Spec. Rep. No. 67; The Iron & Steel Inst., May, 1960, pp. 118-127.

Primary ExaminerW. W. Stallard Attorney, Agent, or Firm-William Anthony Drucker [57] ABSTRACT A soft steel sheet, in which C content is 0.02 to 0.10 percent, manufactured with a continuous annealing process comprising the following steps; heating the cold reduced steel strip up to the range of recrystalli- Mar. 27, 1971 Japan 46-18137 Zetierr temperature to 850C, Slew-Cooling from Said temperature to the range of A point to 600C and [52] U.S. Cl. 148/2, 148/12 then ap g f m said temperature to the nor- [51] Int. Cl C2ld 9/48 mal temperature at a cooling rat f 200C/s t [58] Field of Search 148/12, 12.1, 2, 12.3 l0,000C/s d again ing up t t range of 300C to 530C and keeping at said temperature for at [56] Refer n es Cit d least 10 seconds, lastly cooling to the normal tempera- UNITED STATES PATENTS ture. Such steel sheet is economical in cost because of 3,245,844 4/1966 Weber 148/12 hlgh productwlty' 3,320,099 5/1967 Weber 148/12 8 Claims, 5 Drawing Figures PAIENTEDIET H914 Z I I0 002 004 00s ma 0/0 02 METHOD OF MAKING A DRAWING STEEL SHEET BY CONTINUOUS ANNEALING PROCESS INCLUDING SHELF TREATMENT TI-IEREIN This invention relates to a method of making a drawing steel sheet by continuous annealing process including novel shelf treatment therein, and more particularly making drawing steel sheet with very high productivity.

As well-known for a long time, a continuous annealing treatment was developed as the best suited process for manufacturing tin plate and has been put into practice, because the most suitable hardness for saidtin plate is easily given to the steel strip owing to the rapid heating and cooling in said line. It is, however, wellknown that said hardness obtained by said treatment is too hard to enable the steel sheet to be pressed into various shapes. If a kind of soft steel strip is obtained by said line, there is no doubt that the utility value will be very high as a method of making a commercial cold reduced steel sheet. Therefore, many attempts have been proposed and put into practice. For example, the art as shown in US. Pat. No. 2,832,711 is one of said attempts. This art was developed .to give drawability of some degree to a galvanized steel strip. The features of the above art lie in quenching from the range of about 1,250F and l,300F to below l,000F, following by holding at said annealing temperature for 30 seconds and then holding for at least 30 seconds in the shelf temperature range of 800 to 1,000F. With such a process, the steel sheet becomes softer than that from the ordinary continuous annealing process. According to our experiments, it has been confirmed that the softness and aging resistance, so far attained, are not yet sufficient. The shelf treatment as mentioned above should be further improved if the steel is subjected to press-forming services.

S. Garber, et a]. reports New continuous annealing cycle for blackplate AISI, vol. 200, June, 1962 is another example among said attempts. This art was developed to simplify the ordinary continuous annealing facilities for tin plate. The features of the art lie in the process of heating rapidly to 700C and immediately quenching to about 300C, in which Pb-Bi salt bath is employed, and the over aging for 30 min. as it is coiled. Results equal to those for commercial Temper Universal plate are obtained. Specially, the hardness values of said steel sheet are equal to or better than those from the ordinary cycle. According to our investigations we have found that the softness and aging of steels annealed by the prior art method are not satisfactory. It will be needless to say that productivity based on the above art is inferior to that of an ordinary continuous annealing operation. The steel treated by the above art can be subjected to press-forming services only with difficulty. Thus, a steel which can be subjected to ordinary press-forming services is not yet available.

This invention has been developed to remove the above-mentioned faults and disadvantages. The featurcs of this invention consist in heating the cold reduced steel up to the range of recrystallization temperature to 850C, cooling slowly from said temperature to the range of less than A, point to 600C and then cooling rapidly at the rate of 200C/sec and l0,000C/sec to the normal temperature, and again heating the steel up to the range of 300 to 530C and holding for at least seconds. In such case, when the Further object of this invention is to provide a method of making a drawing steel sheet by a continuous annealing process displaying very high productivity, accordingly, low cost.

Other objects and advantages will be apparent from the following description and accompanying drawings in which:

FIG. 1 is an example of typical annealing cycle of this invention,

FIG. 2 shows an annealing cycle based on this invention in comparison with other cycles,

FIG. 3 is a graph showing the relation between yield stress and carbon content in steel,

FIG. 4 is a graph showing the relation between aging index and carbon content in steel,

FIG. 5 illustrates changing behavior of aging index and yield stress with various velocities.

A typical continuous annealing cycle based on this invention is shown in FIG. 1. T,, T T and T, points on the time-temperature diagram of FIG. 1 show the maximum heating temperature, rapid-cooling starting temperature, the maximum heating temperature for the precipitation treatment and finishing temperature of said precipitation treatment respectively. Said T, point is the maximum heating temperature and is selected from the range of recrystallization temperature to 850C. Next, T point is the rapid-cooling starting temperature and is selected from the range of A, point to 600C. In such a case, when said T, point is more than 850C, the heating will be difficult to be carried out with the ordinary continuous annealing facilities. Even though said heating operation is possible to be realized, the heating is fully within the region of austenization, consequently the lowering of known Lankford Value r will be brought about. Therefore, the upper limit of said heating temperature is 850C. When T point is more than A, point, a known tempering structure is produced partially because of the rapid-cooling and successive precipitation treatment, consequently a harmful influence on ductility of steel tends to occur, while the rapid-cooling from less than 600C results in causing the amount of solution carbon to decrease. Thus, said rapid-cooling starting temperature should be selected from the range of A, point and 600C. The steel strip is cooled slowly from said T, point to T point. And the slow-cooling time should be sufficient to bring a solution of carbon in equilibrium state.

The slowly cooled steel to said T point is rapidly cooled from the temperature to the normal temperature. Such rapid-cooling operation is carried out to increase the amount of super-saturated carbon and number of precipitating nuclei with a velocity of 200C/sec and l0,000C/sec, which is a desirable requirement for the next rapid precipitation treatment of the carbide. Successively again, said steel is heated up to T point. Said T,, point is the maximum heating temperature for precipitation treatment of carbide and is selected from the range of 300 to 530C. T, point is a finishing temperature of said precipitation treatment. Such T point is the same temperature as said T point, i.e., a holding operation, or lower temperature than said T point, i.e., slow cooling operation. In such case, said operation time from T to T should be for at least 10 seconds. The reason that said T and T points are limited as mentioned above is as follows: that is, when said treating temperature is above 530C, the solution degree of carbon tends to increase, and conversely when said temperature is below 300C, said precipitating velocity EXAMPLE 1 Making requirements:

Chemical composition of testing material.

TABLE I Steel C Mn P S N Sol.Al

I 0.005 0.37 0.010 0.019 0.0019 0.029 tr 2 0.012 0.34 0.011 0.021 0.0028 0.034 tr 3 0.023 0.35 0.012 0.017 0.0021 0.036 tr 4 0.035 0.37 0.015 0.019 0.0020 0.027 tr 5 0.043 0.30 0.009 0.018 0.0018 0.023 tr 6 0.060 0.31 0.013 0.023 0.0013 0.035 tr 7 0.075 0.36 0.011 0.023 0.0015 0.036 tr 8 0.091 0.38 0.011 0.020 0.0015 0.037 tr 9 0.108 0.36 0.014 0.021 0.0014 0.036 tr tends to lower. Thus, while the above-mentioned shelf treatment carried out at a temperature between 300C and 530C may secure the same degree of aging index as that of an ordinary batch type annealing process, it will be, however, recommended that the starting temperature of said precipitation treatment is 350C to 450C (T and the finishing temperature is 300C (T Because, by the slow-cooling operation as mentioned above the properties of steel are remarkably improved and the aging index is lowered further. Briefly speaking, this process is characterised by having the solution carbon brought in the equilibrium state during slow-cooling from T, point to T point, supersaturated during rapid-cooling from T point to the normal temperature and precipitated as carbide during successive shelf treatment from T;, point to T point.

As the cooling operation is followed by said shelf treatment, a rapid-cooling operation will be advisable to simplify the treating steps. Thus, this present process may be easily carried out with the same short time as that of an ordinary continuous annealing process.

Chemical composition of the present steel will not be especially specified except carbon content, which is from no less than 0.02 percent to no more than 0.10 percent. The reason that only the carbon content is limited as mentioned above is as follows; when C content is less than 0.02 percent, said aging index does not lower as much as expected because of a small amount of supersaturated carbon after said rapid-cooling, and when C content is more than 0.10 percent, said steel is too hard and unsuitable for press-forming operation.- The annealing process of this invention may be included in the process of ordinary cold reducing steel strip which is a serial process comprising steel making, slabbing or continuous casting, hot rolling, picking and cold reducing. However, when a steel exhibiting lower,

yield point and good drawability is required, said steel should be coiled at a temperature of more than about 630 C following the hot rolling step. It is confirmed that the high temperature coiling step causes crystal Steel of making slabbing hot rolling finishing temperature: 865C coiling temperature: 600C last thickness: 2.8mm Cold reducing after pickling cold reducing rate: 71.4 percent last thickness: 0.8mm Continuous annealing cycle: A, B, C, D and E cycles as shown in FIG. 2

soaking: 710C X 30 sec. rapid cooling rate: 600C/sec. shelf treatment: 400C X 30 see. next air-cooling cycle B: This is a cycle based on this invention.

The maximum heating-up temperature (T,):

800C slow-cooling rate: 5C/sec. rapid-cooling starting temperature (T- 650C rapid-cooling rate: 600C/sec. the maximum reheating-up temperature (T;,):

530C slow-cooling rate: 12C/sec.

finishing temperature of shelf treatment (T 300C next air cooling cycle C:

the maximum heating-up temperature: 780C slow-cooling rate: 5C rapid-cooling starting temperature: 650C rapidcooling rate: 600C/sec. shelf treatment: 400C X 30 see. next air cooling cycle D: soaking: 710C X 30 sec. rapid-cooling rate: 600C/sec. the maximum reheating-up temperature: 500C slow-cooling rate: 12C/sec. finishing temperature of shelf treatment: 300C next air cooling cycle E: This is one of ordinary cycle.

ingly unsuitable material for press-forming. Thus, the

steels having C content of 0.02 to 0.10 percent are the most suitable material for press-forming operations. It

TABLE 11 Yield Yield Tensile Total Aging Annealing Stress Elonga- Strength Elongalndex Lankford Steel Cycle Kg/mm tion(%) Kg/mm tion(%) Kg/mm Value r A 20.9 32.1 46.0 6.2 1.27 B 20.5 0 31.9 46.7. -6.3 1.33 1 C 20.7 0 32.1 46.0 6.0 1.30 D 20.7 0 32.0 46.3 6.1 1.29 E 21.1 0.5 32.3 45.7 6.1 1.30 A 22.0 0 32.8 44.7 6.2 1.22 B 21.6 0 32.7 45.1 6.2 1.27 2 C 21.8 0 32.7 44.9 6.3 1.25 D 22.0 0 32.5 45.2 6.1 1.22 E 21.9 0.6 32.8 45.0 6.5 1.23 A 22.0 0 33.1 44.9 5.5 1.15 B 21.8 0 32.9 45.2 5.6 1.23 3 C 22.0 0 32.9 45.3 5.5 1.21 D 21.9 0 33.3 44.4 5.6 1.14 E 23.7 0.8 33.3 43.1 7.0 1.13 A 23.3 0 33.9 44.9 5.7 1.11 B 22.0 0 33.1 44.7 5.1 1.18 .4 C 22.5 0 33.6 44.7 5.5 1.17 D 22.6 0 33.9 44.9 5.4 1.08 E 25.6 1.3 35.1 42.1 7.2 1.06 A 23.9 0 34.1 44.3 5.2 1.12 B 22.3 0 33.6 45.6 4.9 1.13 5 C 22.9 0 34.4 45.3 5.0 1.18 D 23.2 0 33.9 45.0 5.2 1.08 E 26.8 1.5 35.0 40.9 6.9 1.10 A 25.0 0 34.6 44.1 5.2 1.09 B 23.0 0 33.7 44.6 4.7 1.09 6 C 24.2 0 33.9 44.9 5.0 1.15 D 24.0 0 34.3 44.5 5.0 1.07 E 27.8 1.6 36.0 40.8 7.2 1.10 A 25.2 0 34.9 43.9 4.9 1.06 B 24.1 0 34.6 43.9 4.9 1.03 7 C 24.5 0 35.0 42.7 4.7 1.13 D 24.7 0 35.2 43.6 4.9 1.07 E 28.3 1.2 37.1 39.6 7.6 1.10 A 26.0 0 35.9 43.8 5.6 1.06 B 24.5 0 35.1 43.6 5.5 1.04 8 C 25.3 0 35.5 44.0 5.5 1.09 D 25.1 0 35.7 43.8 5.5 1.05 E 30.6 1.0 38.0 38.5 7.1 1.07 A 27.8 0 36.6 42.1 5.3 1.03 B 26.1 0 36.5 41.9 5.0 1.01 9 C 27.0 0 36.8 42.0 5.2 1.06

D 27.0 0 36.9 42.5 5.1 1.01 E 32.5 1.9 39.7 36.6 6.9 1.00 A 28.2 0 37.9 41.9 5.6 1.00 B 26.7 0 37.8 41.1 5.0 1.03 10 C 27.5 0 38.0 41.7 5.3 1.05 D 27.9 0 37.7 41.5 5.4 1.00 E 33.1 2.6 39.9 35.9 6.8 0.97

In the above Table l. C contents of Steel 3 to Steel 8 among these steels agree with the range specified in this invention. The relation between yield stress and C content exhibiting on steels A. B and E as representatives of said properties is shown in H6. 3 and the relation between aging index and C content is in FIG. 4.

According to the above Tables 1 and 11, it will be well understood that differences of properties depending upon various annealing cycles are scarcely exhibited, and effects owing to shelf treatment are unsettled. in the ease of the steels including C content of less than 0.02 percent. That is. while each of yield stresses of Steel 1 and 2 is relatively low, each aging indexes is considerably high as it is 6.0 and 6.5Kg/mm". Such steels will come into question in practical use. Conversely, when C content is more than 0.10 percent as seen in Steels 9 and 10. said yield stress becomes above 26Kg/mm". It may be estimated as hard quality, accord- -to 8 are as shown in Table 11. According to said Table 11, it is understood that both yield stress and aging index become low. Specially, said yield stress of the steels treated with said cycle B lowers by about 2Kg/mm and said aging index lowers by about 0.5Kg/mm in comparison with that of cycle A. And the properties of steels treated with said cycles C and D are superior to those of said A and inferior to those of said cycle B. Such difference is based on effects of said 8 cycle, i.e., this invention process is cooled slowly from the maximum heating-up temperature to rapid-cooling starting temperature in the range of A, point to 600C, cooled rapidly from said starting temperature to the normal temperature and then cooled slowly the maximum reheating-up temperature to finishing temperature of said shelf treatment.

7 8 EXAMPLE [1 According to the abo e Table 111, it will be wellunderstood that said yield stress and yield strength are Making requirements: lowered in the same manner and said total elongation Chemical composition of testing material. and Lankford value are raised as compared with said the same as that of Example 1. 5 Table 11 in Example 1. It is needless to say that such im- Steel making slabbing provements of said drawability as shown in said Table finishing temperature: 860C 11 depend upon the higher coiling temperature, Le, 70- coiling temperature: 700C 0C, by the side of said Example I, i.c., 600C. It is, silast thickness: 2.8mm multaneously, also understood that the same tendency, Cold reducing after pickling [O as the above Example 1 made clear on relation between the same as that of Example 1. said properties and C content, is exhibited. Thus, with- Continuous annealing out distinction of the kind of steels, when said steel is the same as that of Example 1. coiled with a high temperature of at least 630C said Mechanical properties: properties may be remarkably improved.

TABLE 111 Temper rolling rate: 1.5%

Yie

Yield Tensile Total Aging Annealing Stress Elonga- Strength Elonga- Index Lankford Steel cycle Kg/mm" tion(%) Kg/mm tion(7r) Kg/mm value r B 20.5 0 31.6 45.3 6.0 1.43 2 C 20.7 0 31.3 45.0 6.3 1.40 D 20.9 0 32.0 45.3 6.1 1.37 E 21.9 0.2 32.9 44.1 6.4 1.36

B 22.1 0 33.5 43.8 5.1 1.34 7 C 23.1 0 34.0 43.0 5.5 1.35 D 22.7 0 33.9 44.0 5.4 1.25

EXAMPLE III In this example, Steel V among steels of the above Table l is tested. The object of this test lies in investingating influences to which a cooling rate of from said soaking temperature, i.e., T point, to the normal temperature gives said properties of the steel. The hot rolling and cold reducing requirements on the above steel are the same as that of Example ll. But continuous annealing process was carried out depending upon said Cycle A as shown in Example I. That is;

soaking temperature: 710C X 30 sec.

rapid cooling rate: 17 points of 30,000C/sec to 13C/sec as shown in FIG. 5. shelf treating temperature: 400C X 30 sec. next, air cooling As shown in said FIG. 5, said aging index lowers as said cooling rate increases. However, when the increasing of said rate reaches 200C/sec. said aging index be comes about SKg/mm similar to that of known batch type annealing process and little change occurs on increasing said cooling rate. And in the cooling rate of l0,000C/sec. said yield stress scarcely changes. When said rate is above l0,000C/sec. said yield stress tends to increase. Thus. it will be understood that the cooling rate of this present process, which is from l0,000C/sec to 200C/sec. is the best suited velocity. And moreover,

when said cycle B including the above cooling rate is employed as continuous annealing process, said properties of steel are possible to be further improved, which is as shown in Tables I] and Ill.

As mentioned above, there is no doubt that when a steel strip is treated with this present process said mechanical properties of steel are remarkably improved as composed with that of the ordinary continuous anneal ing process, which is a continuously annealed steel bearable to be press-formed. It should be noted that the industrial utility, i.e., high productivity and good drawability, is given to the continuously annealed steel only with this invention process.

What is claimed is:

1. A method of making a drawing steel sheet comprising the following steps:

making a steel including C content of 0.02 to 0.10

percent, in a continuous annealing stage following an ordinary 10 sla bbing or continuous casting hot-rolling, pickling and cold-reducing,

heating up said cold-reduced steel strip to the range of recrystallization temperature and 850C. slowcooling from said temperature to the range of A point and 600C and then rapid-cooling from said temperature to the normal temperature by a cooling rate of 200C/sec and l0,000C/sec,

shelf-treating in which said strip is reheated up to the range of 300C and 530C, successively cooled slowly from said temperature to at least 300C,

and lastly cooling from said last mentioned temperature to the normal temperature and then tempercooling, in the usual manner.

2. A method of making a drawing steel sheet as set forth in claim 1 wherein coiling operation followed by said hot-rolling is carried out with a temperature of at least 630C.

3. A method of making a drawing steel sheet as set forth in claim 1 wherein said slow-cooling rate following said heating the cold-reduced steel strip is about 5C/sec.

4. A method of making a drawing steel sheet as set forth in claim 1 wherein said reheating temperature is within the range of 350C and 450C.

5. A method of making a drawing steel sheet as set forth in claim 4 wherein said slow-cooling rate follow ing said reheating is about l2C/sec.

6. A method of making a drawing steel sheet as set forth in claim 1 wherein said continuous annealing is carried out with which said strip is heated up at about 800C, cooled slowly to about 650C by the rate of 5C/sec and then cooled rapidly from said temperature to the normal temperature by about 600C/sec and in successive shelf treatment said strip is heated up to 530C and 500C and then cooled slowly to 300C by the rate of about 12C/sec, next air-cooled.

7. A method of making a drawing steel sheet as set forth in claim 6 wherein said continuous annealing process is carried out to the steel strip which is coiled with about 700C in hot rolling stage.

8. A method of making a drawing steel sheet as set forth in claim 7 wherein treating steel strip is an Alkilled steel. 

1. A METHOD OF MAKING A DRAWING STEEL SHEET COMPRISING THE FOLLOWING STEPS: MAKING A STEEL INCLUDING C CONTENT OF 0.02 TO 0.10 PERCENT, IN A CONTINUOUS ANNEALING STAGE FOLLOWING AN ORDINARY SLABBING OR CONTINUOUS CASTING, HOT-ROLLING, PICKING AND COLD-REDUCING, HEATING UP SAID COLD-REDUCED STEEL SRIP TO THE RANGE OF RECRYSTALLIZATION TEMPERATURE AND 850*C, SLOW-COOLING FROM SAID TEMPERATURE TO THE RANG OF A1 POINT AND 6000*C AND THEN RAPID-COOLING FROM SAID TEMPERATURE TO THE NORMAL TEMPERATURE BY A COOLING RATE OF 200*C/SEC AND 10,000*C/SEC, SHELF-TREATING IN WHICH SAID STRIP IS REHEATED UP TO THE RANGE OF 300*C AND 530*C, SUCCESSIVELY COOLED SLOWYL FROM SAID TEMPERATURE TO AT LEAST 300*C, AND LASTLY COOLING FROM SAID LAST MENTIONED TEMPERATURE TO THE NORMAL TEMPERATURE AND THEN TEMPER-COOLING, IN THE USUAL MANNER.
 2. A method of making a drawing steel sheet as set forth in claim 1 wherein coiling operation followed by said hot-rolling is carried out with a temperature of at least 630*C.
 3. A method of making a drawing steel sheet as set forth in claim 1 wherein said slow-cooling rate following said heating the cold-reduced steel strip is about 5*C/sec.
 4. A method of making a drawing steel sheet as set forth in claim 1 wherein said reheating temperature is within the range of 350*C and 450*C.
 5. A method of making a drawing steel sheet as set forth in claim 4 wherein said slow-cooling rate following said reheating is about 12*C/sec.
 6. A method of making a drawing steel sheet as set forth in claim 1 wherein said continuous annealing is carried out with which said strip is heated up at about 800*C, cooled slowly to about 650*C by the rate of 5*C/sec and then cooled rapidly from said temperature to the normal temperature by about 600*C/sec and in successive shelf treatment said strip is heated up to 530*C and 500*C and then cooled slowly to 300*C by the rate of about 12*C/sec, next air-cooled.
 7. A method of making a drawing steel sheet as set forth in claim 6 wherein said continuous annealing process is carried out to the steel strip which is coiled with about 700*C in hot rolling stage.
 8. A method of making a drawing steel sheet as set forth in claim 7 wherein treating steel strip is an Al-killed steel. 